The present invention relates to radio frequency (RF) power amplifier (PA) and front-end modules (FEM). Typical applications targeted for such technologies are cellular phones, Personal Digital assistants (PDA) and wireless connectivity applications such as Digital video Broadcast (DVB-H and DVB-T), Zigbee, wireless local area networks (WLAN) and Ultra wideband applications (UWB). The substantially high transmission power associated with RF communication (for example class 12 global system for mobile communication (GSM) or general packet radio system (GPRS) communication) where the transmit time is half of the total communication time, forces the need to dissipate the energy in a restricted area and increases the difficulty of miniaturization of the package.
A power amplifier module for amplifying RF signals includes a radio frequency amplifier including several RF transistors (PA dice) adapted to accept an input radio frequency signal, biasing signals and control signals (controller die) and to output an amplified version of the input radio frequency signal according to biasing and control signals.
The RF transistors are generally integrated in one or several integrated circuits, such as a Gallium Arsenide (GaAs) IC, Silicon BiCMOS, Silicon Germanium Hetero-junction Bi-polar Transistor, or other CMOS-based technologies. The integrated circuit also includes biasing circuitry, which provides the DC voltage or current and a control circuit allowing the control of the power delivered at the output.
The amplifier ICs are also interconnected to components such as inductors, capacitors resistors or transmission lines used for impedance matching and control of the RF transistors.
A front end module (FEM) is the association of the above power amplifier module (PA) with the matching circuits. The matching circuits are connected to the harmonic filtering circuits. The filtering circuits are connected to the antenna switch. The matching circuits can also be implemented using surface mount devices (SMDs) on any of the following:                (i) Substrate or leads;        (ii) Printed circuit board (PCB) tracks on or in the substrate;        (iii) Back and forth wire-bonding between a substrate and the dice; or        (iv) Integrated passive devices (IPD) on substrate or leads; or        (v) Any combination of the above four implementations.        
The harmonics filtering can be implemented using integrated passive devices on substrate or leads and embedded devices in the substrate. The switch uses a dedicated semiconductor process, such as GaAs, CMOS on Silicon on Insulator (SOI).
As an operating frequency increases, a transistor's characteristics change dramatically. Above 1 MHz, depending upon the transistor, the input and output impedances decrease and become increasingly more reactive. The voltage, current and power gain decrease and there is a greater tendency for signals at the output to feedback to the input through internal capacitance. This leads to a loss of power gain, which is highly undesirable.
Power gain is often used in radio frequency (RF) circuits to emphasize a difference between active and passive circuits. A passive network may have a voltage gain or a current gain, but not both at the same time.
In contrast, the majority of audio-frequency designs involve only minor changes in impedance level and very few impedance changing devices. Voltage gain is a meaningful term under such conditions. At radio frequencies, however, impedance matching is required as, impedance levels throughout a circuit change dramatically. Thus, the only true indication of how good a transistor operates is to calculate its power gain.
For power amplifiers used in RF mobile communication, the power amplifier (PA) performance is often judged based on its Power Added Efficiency (PAE), as it directly impacts, say, the mobile communication unit's talk time. It is known that PAs require excellent grounding in order to obtain an optimum performance in PAE, as well as in gain and output power.
In a dual-band amplifier design, suitable for both low-band and high-band use in digital cellular communication units compliant with the Global System for Mobile (GSM) communications, a PA typically exhibits a performance of 60% PAE in the low GSM band of 850-900 MHz, and 55% PAE in the high-band (global standard) direct communication system (DCS)/personal communication system (PCS) frequencies of 1800-1900 MHz.
European Patent Application titled: “Arrangement and method for Impedance Matching” by Philippe Riondet, Gilles Montoriol, and Jacques Trichet describes a method using wire-bonding and using on chip capacitors for impedance matching of PA and FEM. A known power amplifier packaging design utilises lead-frame inductors.
As illustrated in FIG. 1, such a design 100 has only been implemented for a PA, in this case a dual-band amplifier, whereby a first amplifier 110 is designed for GSM, and a second amplifier 105 is designed for DCS-PCS operation. Furthermore, due to space constraints in being able to build an RF module using such lead-frame technology, the only other component on the die is the power amplifiers' associated control functionality 115. The whole package is configured in a 7×7 mm plastic package.
It is known that it is very difficult to implement high-Q inductors in a small size and at a low cost, as shown by the inductive tracks 120 implemented on the lead-frame package. Furthermore, implementing discrete inductors is impractical, due to the size and cost of such components. Wire bonding has also been shown to provide acceptable high-Q performance of inductors at very low cost. Notably, the lead-frame package described in U.S. Pat. No. 6,621,140 B1 has also been described as a mechanism to achieve a high-Q inductor performance, whilst focusing on achieving the best ‘Q’.
U.S. Pat. No. 6,750,546 B1, by Villanueva et. al. describes an assembly process for a flip chip lead-frame package.
However, a power amplifier designer would ideally like to implement a complete front-end module. Unfortunately, the aforementioned use of high-Q lead-frame inductors 120 proposed in U.S. Pat. No. 6,621,140 B1 occupies about 40% of the PA area, thereby removing any practical possibility to implement a complete front-end module. Furthermore, due to the extensive use of inductive tracks that are required to manufacture a complete RF front-end module, a lead-frame package provides poor signal routeing capability.
PCT application—US2004/0232982 A1 , by Ichitsubo et. al., describes an RF front-end module for wireless communication devices, and is notably focused on the avoidance of using a printed circuit board (PCB)/LTCC and surface mount technologies (SMTs).
However, it is noteworthy that none of the above citations adequately address the aforementioned problem of implementing a RF power module having an improved power performance. In particular, none of the above citations disclose a mechanism that improves power added efficiency, where sufficiently less die size is required to implementing high-Q components, such as inductors, capacitors and RF chokes.
In summary, a key parameter in the design of high performance power amplifiers is the quality of the grounding. Typically, the RF die grounding is realized by soldering the die 205, using tin lead solder, or gold tin eutectic solder), directly on a metallic flange heatsink 220, as illustrated in the circuit arrangement 200 of FIG. 2. The active die is operably coupled to substantially co-located PCBS 210 via wire-bonds 215. This known art allows the best thermal contact between the die active area 205 and the best electrical contact for the grounding. It is also well known that a resistive or inductive grounding of the RF power device generates fast degradation of power gain and power added efficiency.
However, a similar structure 300 is illustrated in FIG. 3, which has been widely used as a low cost structure. Here, an active die 305 is directly coupled to surface mounted components on a PCB 310 via wire-bonds 315. A primary weakness of this structure is a significantly worse grounding, as compared to the RF die grounding of soldering directly on a metallic flange as shown in FIG. 2. This remains the case even if there is a large number of via holes 320 underneath the active die 305.
Thus, a need exists for a low cost, lead-frame packaging technology that allows the PA die to exhibit an improved performance, whilst offering high routeability and ease of implementing high-Q components.